Kinetics of the adsorption and desorption of radionuclides of Co, Mn, Cs, Fe, Ag and Cd in freshwater systems: experimental and modelling approaches

The sorption and release kinetics of 54Mn, 58Co, 59Fe, 109Cd, 110mAg and 134Cs by freshwater suspended particles were investigated to better identify the biogeochemical processes involved and to obtain suitable data for improving models describing radionuclide migration in freshwater streams. In order to observe any seasonal variability in the interaction of radionuclides with natural particles, experiments were performed both in winter and in summer during a phytoplanktonic bloom. Two kinetic models are compared in this paper: the "one-step reversible" model, based on the hypothesis that the transfer of radionuclides between water and suspended solids is governed by a reversible reaction, and the "two-successive-step reversible" model, which assumes two distinct types of sites or reactions on the solid phase. The "one-step reversible" model is generally unable to describe properly the exchange kinetics; this result shows that at least two processes are generally involved in radionuclide exchange between water and suspended particles. On the contrary, a model involving the existence of two successive reversible reactions properly simultes both the sorption and release kinetics. The determination of the kinetic coefficients allows quantitative assessment of the relative importance and kinetics of the processes. In particular, it has been shown that, except for Cs and Cd, major fractions of the radionuclides are associated at equilibrium with particulate sites involving strong interactions. The kinetics to reach this equilibrium depend on seasonal conditions, especially for Co and Mn: the transfer of Co and Mn to particulate sites involving strong interactions is much slower in winter. The distribution of the radionuclides between water and particulate sites involving weak interactions also shows seasonal variations for Co, Mn, Fe and Ag: the capacity to associate radionuclides is much higher in summer for Co and Mn, while the inverse tendency is observed for Fe and Ag. For Cs and Cd, no significant seasonal differences were observed.     

Soil migration and plant uptake of technetium from a fluctuating water table

Soil columns (50x15 cm) were used to determine the potential for 95mTc (as a surrogate for 99Tc which is an important component of some radioactive waste) to migrate from a contaminated, fluctuating water table, through sandy loam soil and into perennial ryegrass. Upward migration was significantly retarded with, generally, only the bottom few centimetres of soil becoming contaminated over the 6 months of the experiment. This is thought to have been due to the presence of anoxic conditions within the water table leading to the reduction of pertechnetate to Tc(IV) species which are relatively insoluble. However, some evidence of very slow upward migration over time was found. Only a small and inconsistent transfer of activity into the perennial ryegrass was observed. Whilst these observations would suggest that 99Tc is less important than radionuclides such as 129I and 36Cl in terms of the risk associated with radioactive waste disposal, the potential for a slow upward migration, and/or a pulse-release following the re-oxidation of reduced soil in which 99Tc has accumulated should not be overlooked.     

A review and model assessment of (32)P and (33)P uptake to biota in freshwater systems

Bioaccumulation of key short-lived radionuclides such as ¹³¹I and ^32,33P may be over-estimated since concentration ratios (CRs) are often based on values for the corresponding stable isotope which do not account for radioactive decay during uptake via the food chain. This study presents estimates for bioaccumulation of radioactive phosphorus which account for both radioactive decay and varying ambient levels of stable P in the environment. Recommended interim CR values for radioactive forms of P as a function of bioavailable stable phosphorus in the water body are presented. Values of CR are presented for three different trophic levels of the aquatic food chain; foodstuffs from all three trophic levels may potentially be consumed by humans. It is concluded that current recommended values of the CR are likely to be significantly over-estimated for radioactive phosphorus in many freshwater systems, particularly lowland rivers. Further research is recommended to field-validate these models and assess their uncertainty. The relative importance of food-chain uptake and direct uptake from water are also assessed from a review of the literature. It can be concluded that food-chain uptake is the dominant accumulation pathway in fish and hence accumulation factors for radioactive phosphorus in farmed fish are likely to be significantly lower than those for wild fish.     

Upward movement of tritium from contaminated groundwaters: a numerical analysis

This paper describes a research-oriented modelling exercise that addresses the problem of assessing the movement of tritium from a contaminated perched aquifer to the land surface. Participants were provided with information on water table depth, soil characteristics, hourly meteorological and evapotranspiration data. They were asked to predict the upward migration of tritium through the unsaturated soil into the atmosphere. Eight different numerical models were used to calculate the movement of tritium. The modelling results agree within a factor of two, if very small time and space increments are used. The agreement is not so good when the near-surface soil becomes dry. The modelling of the alternate upward and downward transport of tritium close to the ground surface generally requires rather complex models and detailed input because tritium concentration varies sharply over short distances and is very sensitive to many interactive factors including rainfall amount, evapotranspiration rate, rooting depth and water table position.     

A comparison of the soil migration and plant uptake of radioactive chlorine and iodine from contaminated groundwater

A 6-month soil column experiment was conducted to compare the upward migration and plant uptake of radiochlorine and radioiodine from shallow, near-surface contaminated water tables. Both fixed and fluctuating water tables were studied. After 6 months, (36)Cl activity concentrations were relatively uniform throughout the soil profile apart from an accumulation at the soil surface, which was especially marked under a fluctuating water table scenario. In contrast, (125)I (a surrogate for (129)I) tended to accumulate at the boundary between the anoxic conditions at the base of the column and the oxic conditions above, due to its redox-dependent sorption behaviour. The uptake of (36)Cl by perennial ryegrass was much greater than that of (125)I due to its greater migration into the rooting zone and its ready availability in soil solution. In the context of radioactive waste disposal, where these radionuclides may potentially be released into groundwater, (36)Cl would be expected to present a greater potential for contamination of the biosphere than (129)I.     

Test of existing mathematical models for atmospheric resuspension of radionuclides

Atmospheric resuspension of radionuclides can be a secondary source of contamination after a release has stopped, as well as a source of contamination for people and areas not exposed to the original release. A test scenario based on measurements collected after the Chernobyl accident was used to evaluate existing mathematical models for contaminant resuspension from soil, to examine resuspension processes on both local and regional scales, and to investigate the importance of seasonal variations of these processes. Model predictions from 15 participants were compared with measured air concentrations and resuspension factors to investigate and explain the discrepancies both among model predictions and between model predictions and observations and thus to evaluate the predictive capabilities and drawbacks of commonly used generic resuspension models. The empirical models tested can give predictions within an order of magnitude of observations or better if adequately calibrated for site-specific conditions, but they do not describe the process-level events or account for spatial heterogeneity or temporal variations.     

Vertical distribution, migration rates, and model comparison of actinium in a semi-arid environment

Vertical soil characterization and migration of radionuclides were investigated at four radioactively contaminated sites on Kirtland Air Force Base (KAFB), New Mexico to determine the vertical downward migration of radionuclides in a semi-arid environment. The surface soils (0-15 cm) were intentionally contaminated with Brazilian sludge (containing (232)Thorium and other radionuclides) approximately 40 years ago, in order to simulate the conditions resulting from a nuclear weapons accident. Site grading consisted of manually raking or machine disking the sludge. The majority of the radioactivity was found in the top 15 cm of soil, with retention ranging from 69 to 88%. Two models, a compartment diffusion model and leach rate model, were evaluated to determine their capabilities and limitations in predicting radionuclide behavior. The migration rates of actinium were calculated with the diffusion compartment and the leach rate models for all sites, and ranged from 0.009 to 0.1 cm/yr increasing with depth. The migration rates calculated with the leach rate models were similar to those using the diffusion compartment model and did not increase with depth (0.045-0.076, 0.0 cm/yr). The research found that the physical and chemical properties governing transport processes of water and solutes in soil provide a valid radionuclide transport model. The evaluation also showed that the physical model has fewer limitations and may be more applicable to this environment.     

Modelling and experimental studies on the transfer of radionuclides to fruit

Although fruit is an important component of the diet, the extent to which it contributes to radiological exposure remains unclear, partially as a consequence of uncertainties in models and data used to assess transfer of radionuclides in the food chain. A Fruits Working Group operated as part of the IAEA BIOMASS (BIOsphere Modelling and ASSessment) programme from 1997 to 2000, with the aim of improving the robustness of the models that are used for radiological assessment. The Group completed a number of modelling and experimental activities including: (i) a review of experimental, field and modelling information on the transfer of radionuclides to fruit; (ii) discussion of recently completed or ongoing experimental studies; (iii) development of a database on the transfer of radionuclides to fruit; (iv) development of a conceptual model for fruit and (v) two model intercomparison studies and a model validation study. The Group achieved significant advances in understanding the processes involved in transfer of radionuclides to fruit. The work demonstrated that further experimental and modelling studies are required to ensure that the current generation of models can be applied to a wide range of scenarios.     

Radiological risk assessment and biosphere modelling for radioactive waste disposal in Switzerland

Long-term safety assessments for geological disposal of radioactive waste in Switzerland involve the demonstration that the annual radiation dose to humans due to the potential release of radionuclides from the waste repository into the biosphere will not exceed the regulatory limit of 0.1 mSv. Here, we describe the simple but robust approach used by Nagra (Swiss National Cooperative for the Disposal of Radioactive Waste) to quantify the dose to humans as a result to time-dependent release of radionuclides from the geosphere into the biosphere. The model calculates the concentrations of radionuclides in different terrestrial and aquatic compartments of the surface environment. The fluxes of water and solids within the environment are the drivers for the exchange of radionuclides between these compartments. The calculated radionuclide concentrations in the biosphere are then used to estimate the radiation doses to humans due to various exposure paths (e.g. ingestion of radionuclides via drinking water and food, inhalation of radionuclides, external irradiation from radionuclides in soils). In this paper we also discuss recent new achievements and planned future work.     

Transport and fate of radionuclides in aquatic environments--the use of ecosystem modelling for exposure assessments of nuclear facilities

In safety assessments of nuclear facilities, a wide range of radioactive isotopes and their potential hazard to a large assortment of organisms and ecosystem types over long time scales need to be considered. Models used for these purposes have typically employed approaches based on generic reference organisms, stylised environments and transfer functions for biological uptake exclusively based on bioconcentration factors (BCFs). These models are of non-mechanistic nature and involve no understanding of uptake and transport processes in the environment, which is a severe limitation when assessing real ecosystems. In this paper, ecosystem models are suggested as a method to include site-specific data and to facilitate the modelling of dynamic systems. An aquatic ecosystem model for the environmental transport of radionuclides is presented and discussed. With this model, driven and constrained by site-specific carbon dynamics and three radionuclide specific mechanisms: (i) radionuclide uptake by plants, (ii) excretion by animals, and (iii) adsorption to organic surfaces, it was possible to estimate the radionuclide concentrations in all components of the modelled ecosystem with only two radionuclide specific input parameters (BCF for plants and Kd). The importance of radionuclide specific mechanisms for the exposure to organisms was examined, and probabilistic and sensitivity analyses to assess the uncertainties related to ecosystem input parameters were performed. Verification of the model suggests that this model produces analogous results to empirically derived data for more than 20 different radionuclides.     

A modelling study on 137Cs and 239,240Pu behaviour in the Alborán Sea, western Mediterranean

A model for simulating the dispersion processes of 137Cs and 239,240Pu in the Alborán Sea is described. The model consists of two hydrodynamic models: a 2D depth-averaged model and a two-layer model which provide tidal and geostrophic currents, respectively; a sediment transport model which provides suspended particle concentrations and sedimentation rates over the domain; and the radionuclide dispersion model including interactions of dissolved radionuclides with suspended particles and bed sediments. These processes are formulated using kinetic transfer coefficients. The hydrodynamic and sediment models are run and validated in advance, and their results are then used to simulate the dispersion of 137Cs and 239,240Pu, which are introduced from atmospheric fallout. Radionuclide concentrations in the water column and distributions in bed sediments have been compared with measurements in the sea. Both set of data are, in general, in agreement. The model has also been applied to calculate radionuclide fluxes through the Strait of Gibraltar. These computed fluxes have been compared with previous estimations as well.     

Modelling multiple dispersion of radionuclides through the environment

The migration of a contaminant through the environment is the result of the transport by a variety of biotic and abiotic carriers which move according to different dispersion mechanisms. Consequently, the patterns of the distribution of a pollutant in the environment cannot be ever explained on the basis of a single migration process or assuming that the concentrations of contaminant in the different kinds of carriers quickly reach the equilibrium condition. The present work discusses two examples (wash-off from catchments and transport through soils of radionuclides) that clearly demonstrate the inadequacy of “single dispersion” models to predict these patterns. On the contrary, models based on multiple dispersion can successfully simulate the particular features of the above mentioned processes. It was demonstrated that the time behaviour of radionuclide migration rates from catchment of different rivers vary within small ranges as a consequence of multiple dispersion. This result can be useful for the development of generic predictive models.     

Computerised Decision Support Systems for the management of freshwater radioecological emergencies: assessment of the state-of-the-art with respect to the experiences and needs of end-users

Assessment of the environmental and radiological consequences of a nuclear accident requires the management of a great deal of data and information as well as the use of predictive models. Computerised Decision Support Systems (CDSS) are essential tools for this kind of complex assessment and for assisting experts with a rational decision process. The present work focuses on the assessment of the main features of selected state-of-the-art CDSS for off-site management of freshwater ecosystems contaminated by radionuclides. This study involved both developers and end-users of the assessed CDSS and was based on practical customisation exercises, installation and application of the decision systems. Potential end-users can benefit from the availability of several ready-to-use CDSS that allow one to run different kinds of models aimed at predicting the behaviour of radionuclides in aquatic ecosystems, evaluating doses to humans, assessing the effectiveness of different kinds of environmental management interventions and ranking these interventions, accounting for their social, economic and environmental impacts. As a result of the present assessment, the importance of CDSS “integration” became apparent: in many circumstances, different CDSS can be used as complementary tools for the decision-making process. The results of this assessment can also be useful for the future development and improvement of the CDSS.     

Effect of liming on the behaviour of (90)Sr and (137)Cs in a lake ecosystem

Liming of lakes is considered one possible remedial action to reduce the accumulation of radionuclides into fish in the case of a radiological accident. These responses were tested in field conditions in a small acidified lake that was divided into two parts: one limed with CaCO₃ and the other half left as an unlimed control. The transfer of ⁹⁰Sr from water into fish decreased on average by 50% during the first year after liming. However, at the same time the ⁹⁰Sr concentration in water increased, reaching a maximum within 6 months after liming. Approximately 50% more ⁹⁰Sr was detected in water in the limed part of the lake than on control side during the first year. ⁹⁰Sr was most probably released from the sediment as the Ca concentration and pH of the water increased. As a result of these two processes, which counterbalanced each other (increased release of ⁹⁰Sr into water from sediment and decreased transfer of ⁹⁰Sr from water into fish), the ⁹⁰Sr concentration in fish did not notably differ between the limed and control sides of the lake. Liming may only be suitable as a remedial action if carried out immediately after a radiological accident, before significant amounts of radionuclides have been deposited in lake sediments. In the case of ¹³⁷Cs, the effect of liming was less pronounced. ¹³⁷Cs activity concentration in water increased in the first year by 20% and uptake by fish decreased by 20%.     

Two-dimensional numerical modeling of Sr transport in an unsaturated Chinese loess under artificial sprinkling

⁹⁰Sr is a fission byproduct of uranium and plutonium, and it presents a major health problem in the environment. A field test on the transport of various radionuclides including ⁹⁰Sr in an unsaturated Chinese loess was conducted under artificial rain conditions from July 1997 to August 2000. The vertical concentration distribution of ⁹⁰Sr displayed an unusual profile of double concentration peaks, which were separated by a thin (0.7 cm) source layer. In order to interpret the double-peak concentration profile, the transport of ³H and ⁹⁰Sr in the unsaturated Chinese loess under artificial sprinkling conditions was simulated using WATERM, a numerical code for simulating flow field, and NESOR, also a numerical code but for simulating nuclide migration. The models were able to adequately simulate the double-peak concentration profile. The observation suggested that the fine arenaceous quartz layer, though 0.7 cm thick, formed a capillary barrier together with the local loess, which prevented water from penetrating. A significant discrepancy was observed between the model-fitted distribution coefficient (K_d) of ⁹⁰Sr and that determined from independent laboratory experiments, which can be attributed to a number of factors such as the capillary barrier effect, solution-to-solid ratio and soil water content. Therefore, when the model is used for predictive purposes where K_d is used as an input parameter, K_d must be determined under well controlled conditions by taking into account these factors as well as the heterogeneity in the field.     

Uptake of dry-deposited radionuclides in Fucus—A field study after the Chernobyl accident

The dry deposition of various radionuclides emanating from the plume from the Chernobyl reactor accident was measured at two locations in southern Sweden at the beginning of May, 1986. Samples of Fucus, taken at or near these locations, were also analysed. No precipitation had fallen during the time between the accident and the time of sampling. The ratios between activity concentrations in Fucus and dry depositions on the water surface have been calculated. For any specific radionuclide, this ratio was found to be the same at the two locations, after differences in salinity and in uptake between F. vesiculosus and F. serratus had been taken into account.     

Radionuclide transfer from soil to fruit

The available literature on the transfer of radionuclides from soil to fruit has been reviewed with the aim of identifying the main variables and processes affecting the behaviour of radionuclides in fruit plants. Where available, data for transfer of radionuclides from soil to other components of fruit plant have also been collected, to help in understanding the processes of translocation and storage in perennial plants. Soil-to-fruit transfer factors were derived from agricultural ecosystems, both from temperate and subtropical or tropical zones. Aggregated transfer factors have also been collected from natural or semi-natural ecosystems. The data concern numerous fruits and various radionuclides. Soil-to-fruit transfer is nuclide specific. The variability for a given radionuclide is first of all ascribable to the different properties of soils. Fruit plant species are very heterogeneous, varying from woody trees and shrubs to herbaceous plants. In temperate areas the soil-to-fruit transfer is higher in woody trees for caesium and in shrubs for strontium. Significant differences between the values obtained in temperate and subtropical and tropical regions do not necessarily imply that they are ascribable to climate. Transfer factors for caesium are higher in subtropical and tropical fruits, while those for strontium, as well as for plutonium and americium, in the same fruits, are lower; these results can be interpreted taking into account different soil characteristics.     

14C and 210Pb in NE atlantic sediments: Evidence of biological reworking in the context of radioactive waste disposal

Scavenging by the seabed and by sedimentary particles in the deep ocean may have a significant effect on the removal of artificial radionuclides released to the water column as a result of radioactive waste disposal operations. Biological activity in the upper layers of the sediment column will enhance the rate of removal for those particle-reactive radionuclides with a short half-life relative to the turnover time of the upper mixed layer. For longer-lived radionuclides the rate of sediment accumulation will determine the ultimate rate of removal.The rate of sediment accumulation and extent of biological mixing of deep-sea sediment from three areas of the NE Atlantic Ocean have been investigated using ¹⁴C and ²¹⁰Pb data. Treatment of the radiocarbon with a simple box model provided estimates of sedimentation rate (ω), mixed layer depth (L), mixed layer age (T_ML) and the age of material arriving at the surface (T₀), which were broadly similar to previously published values from other ocean basins. Box cores from the Iberia Abyssal Plain, Madeira Abyssal Plain and from the NEA low-level radioactive waste dumpsite yielded sedimentation rates in the range 0.8 to 2.2 cm ky^?1 over the upper 16–25 cm. Continuous particle mixing appears to be taking place to a depth of 4 to 6 cm below the present sediment-water interface. Closely spaced vertical sampling of core 161-2 for ²¹⁰Pb allowed a biodiffusion coefficient (D_B) to be calculated (4 × 10^?9cm²s^?1), treating bioturbations as a diffusive term and sedimentations as an advective term in a simple mathematical model. In general, values of D_B in deep ocean sediments fall within the range 1–10 × 10^?9cm²s^?1, two orders of magnitude lower than nearshore values. From a review of published data it is concluded that biological mixing takes place extensively in the deep ocean; it appears to be fairly constant on a basin-wide scale and amenable to incorporation in mathematical models of radionuclide behaviour in the water column.     

Upward movement of plutonium to surface sediments during an 11-year field study

An 11-year lysimeter study was established to monitor the movement of Pu through vadose zone sediments. Sediment Pu concentrations as a function of depth indicated that some Pu moved upward from the buried source material. Subsequent numerical modeling suggested that the upward movement was largely the result of invading grasses taking up the Pu and translocating it upward. The objective of this study was to determine if the Pu of surface sediments originated from atmosphere fallout or from the buried lysimeter source material (weapons-grade Pu), providing additional evidence that plants were involved in the upward migration of Pu. The ²⁴⁰Pu/²³⁹Pu and ²⁴²Pu/²³⁹Pu atomic fraction ratios of the lysimeter surface sediments, as determined by Thermal Ionization Mass Spectroscopy (TIMS), were 0.063 and 0.00045, respectively; consistent with the signatures of the weapons-grade Pu. Our numerical simulations indicate that because plants create a large water flux, small concentrations over multiple years may result in a measurable accumulation of Pu on the ground surface. These results may have implications on the conceptual model for calculating risk associated with long-term stewardship and monitored natural attenuation management of Pu contaminated subsurface and surface sediments.     

Review and assessment of models used to predict the fate of radionuclides in lakes

A variety of models for predicting the behaviour of radionuclides in fresh water ecosystems have been developed and tested during recent decades within the framework of many international research projects. These models have been implemented in Computerised Decision Support Systems (CDSS) for assisting the appropriate management of fresh water bodies contaminated by radionuclides. The assessment of the state-of-the-art and the consolidation of these CDSSs has been envisaged, by the scientific community, as a primary necessity for the rationalisation of the sector. The classification of the approaches of the various models, the determination of their essential features, the identification of similarities and differences among them and the definition of their application domains are all essential for the harmonisation of the existing CDSSs and for the possible development and improvement of reference models that can be widely applied in different environmental conditions. The present paper summarises the results of the assessment and evaluation of models for predicting the behaviour of radionuclides in lacustrine ecosystems. Such models were developed and tested within major projects financed by the European Commission during its 4th Framework Programme (1994-1998). The work done during the recent decades by many modellers at an international level has produced some consolidated results that are widely accepted by most experts. Nevertheless, some new results have arisen from recent studies and certain model improvements are still necessary.